Audio signal compression

ABSTRACT

Estimating a compression gain obtainable in compressing a given audio signal, comprising extracting a signal power in a selected frequency band of the given audio signal, and obtaining an estimation of the compression gain by correlation with the extracted signal power.

[0001] International Patent Application WO 98/16014 discloses a datacompression apparatus for data compressing an audio signal. The datacompression apparatus comprises an input terminal for receiving theaudio signal, a 1-bit A/D converter for A/D converting the audio signalso as to obtain a bitstream signal, a lossless coder for carrying out alossless data compression step on the bit-stream signal so as to obtaina data compressed bit-stream signal, and an output terminal forsupplying the data compressed bit-stream signal. Further, a recordingapparatus and a transmitter apparatus comprising the data compressionapparatus are disclosed. In addition, a data expansion apparatus fordata expanding the data compressed bit-stream signal supplied by thedata compression apparatus is disclosed, as well as a reproducingapparatus and a receiver apparatus comprising the data expansionapparatus.

[0002] It is an object of the invention to provide advantageouscompression gain estimation in audio compressing. To this end theinvention provides a method and device for estimating a compressiongain, a recording apparatus and a transmitter as defined in theindependent claims. Advantageous embodiments are defined in thedependent claims.

[0003] A compression gain estimation can be used in authoring and/orediting of audio signal sequences such as music pieces. It may be usedfor control of signal processing parameters, such as adaptive control ofthe compression ratio by compressive encoding, to allow recording of thecontent of e.g. a music piece within the limited maximum storage contentof a recording medium such as a Super Audio Compact Disc (SACD) byproviding a quick estimate of the amount of data that can be stored orrecorded on the storage or recording medium.

[0004] Theoretically, to produce such an estimate it could be envisagedto use the average actual compression ratio or coding gain for a smallfraction of an audio signal sequence such as a piece of music as anindication of the compression ratio for the entire sequence. Such anapproach would suffer, however, from the problem that typical audiosignal sequences in the form of music pieces have a rather widevariation of compression ratios or coding gains with significantshort-time correlations, whereby a very significant fraction of thesignal sequence would have to be used for obtaining a compression ratioestimate having the required level of precision or accuracy. Due to therequired time for computation such a solution would in practice not beacceptable.

[0005] According to a first aspect of the invention an advantageousestimate of the obtainable compression gain is obtained by extraction ofa signal power contained in a selected frequency band of the audiosignal and obtaining said estimate by correlation with the extractedsignal power. This aspect of the invention is based on the insight thata quick estimate of the compression ratio or coding gain can be obtainedby using a correlation between the signal power of an audio signal andthe compression gain. This aspect is especially advantageous inlosslessly compressing 1 bit bit-stream signals e.g. in Direct StreamDigital format.

[0006] It is noted that from standard PCM coding it is known that theattainable coding gain is directly related to the degree of structure(flatness) in the signal power. However, in the above aspect of theinvention the signal power itself is used, not its structure.

[0007] In the following the invention will be further explained withreference to the accompanying drawings, in which

[0008]FIG. 1 is a graphic representation of the relationship betweencompression gain and signal power in a selected frequency band of anaudio signal,

[0009]FIG. 2 is a simplified block diagram of a digital audio signalrecording or transmission chain incorporating an embodiment of a signalprocessing apparatus according to the invention, and

[0010]FIG. 3 a simplified topology diagram of a 5th order sigma-deltamodulator for use in the signal processor in the block diagram in FIG.2.

[0011] Whereas investigations for a lossless encoder have demonstratedthat in the audio signal band itself, e.g. up to 20 kHz a relativelyflat response curve exists for the compression ratio as function of thesignal power, a significant steep response of the compression gain cg independence of the signal power sp is obtained for a signal band outsidethe audio signal band, e.g. as illustrated in FIG. 1 for the signal bandfrom 20 to 50 kHz.

[0012] In the signal processing apparatus represented by the blockdiagram in FIG. 2 this distinctive correlation is used to obtain anestimate of the compression ratio or coding gain obtained by losslessencoding. An input audio signal, which may be an analog or a digitalsignal is supplied to a signal processor 1, in which the input audiosignal is processed in dependence of at least one variable processingparameter determined by a parameter control device 2. In the exampleillustrated, the control signal generated by the control device 2controls adaptation of an adaptive sigma-delta modulator 3, whichproduces a data-stream signal in the so-called DSD (Direct StreamDigital) format, such as a 1 bit bit-stream signal, as output signalfrom the signal processor 1 to be sup-plied to the lossless encoder 4.The compression gain obtained in the encoder 4 may be significantlyaffected by variation of the variable parameter used for control of thesignal processor 1, which in result may strongly affect the amount ofdata that can be recorded or stored by the recording medium, which isrepresented in FIG. 2 by a SACD recording on a DVD disc 5. Reference ismade to co-pending application of the same applicant entitled ‘Audiosignal processing’ having the same priority date as this application.

[0013] According to an embodiment of the invention a correlation asrepresented in FIG. 1 between the signal power of the bit-stream signalin the DSD format and the compression gain is used to provide a quickand accurate estimate of the coding gain. As shown the signal power maybe extracted from the bit-stream signal by an extraction and correlationdevice 6 connected with the output from the signal processor 1 andsupplying the compression ratio or coding gain estimate as an inputcontrol signal to the parameter control device 2.

[0014] As shown the parameter controlled component of the signalprocessor 1 may typically be an adaptive sigma-delta modulator 3 andseveral approaches can be used for adaptation or modification of thestructure of such a sigma-delta modulator such as shifting betweensigma-delta modulator structures of different order or creation ofstructure in the high frequency noise of the modulator.

[0015] Thus, by shifting between modulator structures of the 3rd, 5th or7th order compression ratios may be obtained ranging typically fromabout 3.7 for a 3rd order structure down to only 2.3 or lower for a 7thorder structure and this can be used to shape the amount of data in thecompressed bit-stream signal from the encoder 4 to fit within thelimited record-ing or storage capacity of the recording medium such asthe DVD disc 5.

[0016] The diagram in FIG. 3 shows a preferred topology of a 5th ordersigma-delta modulator for use in the signal processor 1 in FIG. 2. Theillustrated topology is based on a multiple resonator structure, inwhich the coefficients c1, c2, . . . c5 in the feed-back loops ofresonators R1, R2, . . . R5 determine the poles of the loop filters (orzeroes of the noise transfer function). Whereas the illustrated topologyis for a 5th order modulator the same topology may be used for a 7thorder modulator just by adding another resonator structure.

[0017] In ordinary design of a such a sigma-delta modulator the poleswill normally be positioned in the audio band, According to theinvention it is preferred, however, to have at least one pole positionedoutside the audio band to create additional structure in the—otherwisealmost flat—high frequency part of the sigma-delta spectrum.

[0018] Thus, whereas in standard designs of sigma-delta modulators thepoles are typically positioned at 8.7, 15.7 and 19.5 kHz, the last poleis, in accordance with the invention, preferably shifted from the 20 kHzregion to a higher frequency region, e.g. around 300 kHz or higher,which may lead to a significant increase of compression gain. Althoughthis may be accompanied by a slight decrease of the signal-to-noiseperformance, this would normally be quite acceptable, because the extranoise is introduced on the high side of the frequency band, where thehuman ear is less sensitive.

[0019] The shifting of the pole position from the 20 kHz region towardshigher frequencies can be effected by addition of a separate extra bandpass filter to the existing modulator structure, e.g. in parallel to thelow-pass loop filter. By use of a 2nd order Butter-worth band passfilter for such a parallel filter a significant increase of compressiongain can be realized with the resulting modulator remaining stable forlarge inputs and the signal-to-noise performance in the audio bandremaining virtually unchanged with respect to an unmodified modulator.

[0020] To take account of the fact that shifting between differentsigma-delta modulator structures may cause variation in the relationbetween the signal power in a certain frequency band of the bit-streamsignal and the compression ratio or coding gain and may thereby affectthe correlation used as basis for the estimation of the compressionratio, it may be advantageous to use an initial calibration process tofix the correlation.

[0021] For this purpose a fixed sigma-delta modulator may beincorporated as a final processing device be-fore the lossless encoder 4as shown by the block 7 drawn in dashed lines in FIG. 2. Thereby, thesignal power should preferably be extracted from the signal suppliedfrom the fixed sigma-delta modulator 7 to the lossless encoder 4.

[0022] Whereas in the signal processor embodiment described above andshown in FIG. 2 an adaptive sigma-delta modulator is used to adapt ormodify the compression ratio or coding gain in response to thecorrelation obtained from the signal power of the bit-stream signalother approaches, such as reduction of the signal level of incorporationof a bandwidth limiting low-pass filter in the signal processor before asigma-delta modulator, that may be adaptive or fixed, may be used eitherseparately or in combination.

[0023] Moreover, a signal processing apparatus for carrying out themethod according to embodiments of the invention need not by itselfcomprise compressive encoding means, in as much as the estimate of thecompression ratio obtainable in subsequent compressive encoding may beused separately, e.g. in a studio environment for pure evaluation,formatting, authoring and/or editing of an audio signal sequence.

[0024] It should be noted that the above-mentioned embodimentsillustrate rather than limit the invention, and that those skilled inthe art will be able to design many alternative embodiments withoutdeparting from the scope of the appended claims. In the claims, anyreference signs placed between parentheses shall not be construed aslimiting the claim. The word ‘comprising’ does not exclude the presenceof other elements or steps than those listed in a claim. The inventioncan be implemented by means of hardware comprising several distinctelements, and by means of a suitably programmed computer. In a deviceclaim enumerating several means, several of these means can be embodiedby one and the same item of hardware. The mere fact that certainmeasures are recited in mutually different dependent claims does notindicate that a combination of these measures cannot be used toadvantage.

1. A method of estimating a compression gain obtainable in compressing agiven audio signal, the method comprising the steps of: extracting asignal power in a selected frequency band of the given audio signal, andobtaining an estimation of the compression gain by correlation with theextracted signal power.
 2. A method as claimed in claim 1, wherein theaudio signal is a 1-bit noise shaped digital signal.
 3. A method asclaimed in claim 1, wherein the compressing is lossless.
 4. A method asclaimed in claim 1, wherein said selected frequency band is above 20kHz.
 5. A method as claimed in claim 4, wherein said selected frequencyband is from 20 to 50 kH.
 6. A method of recording an audio signal on arecording medium, the method comprising the steps of: compressing theaudio signal to obtain a compressed audio signal, recording thecompressed audio signal on the recording medium, the method furthercomprising: estimating a compression gain obtainable in the compressingstep by extracting a signal power in a selected frequency band of theaudio signal, and obtaining an estimation of the compression gain bycorrelation with the extracted signal power, controlling saidcompressing depending on said estimated compression gain.
 7. A method asclaimed in claim 6, wherein said compressing step comprises: convertingthe audio signal into a digital signal by an adaptive noise-shapingmodulation in response to the estimated compression gain,
 8. A method oftransmitting an audio signal, the method comprising the steps of:compressing the audio signal to obtain a compressed audio signal,transmitting the compressed audio signal over a transmission medium, themethod further comprising: estimating a compression gain obtainable inthe compressing step by extracting a signal power in a selectedfrequency band of the audio signal, and obtaining an estimation of thecompression gain by correlation with the extracted signal power,controlling said compressing depending on said estimated compressiongain.
 9. A device for estimating a compression gain obtainable incompressing a given audio signal, the device comprising: means forextracting a signal power in a selected frequency band of the givenaudio signal, and means for obtaining an estimation of the compressiongain by correlation with the extracted signal power.
 10. A recordingapparatus for recording an audio signal on a recording medium, therecording apparatus comprising: means for compressing the audio signalto obtain a compressed audio signal, means for recording the compressedaudio signal on the recording medium, the recording apparatus furthercomprising: a device as claimed in claim 9 for estimating a compressiongain obtainable in the compressing, means for controlling saidcompressing depending on said estimated compression gain.
 11. Atransmitter for transmitting an audio signal, the transmittercomprising: means for compressing the audio signal to obtain acompressed audio signal, means for transmitting the compressed audiosignal over a transmission medium, the transmitter further comprising: adevice as claimed in claim 9 for estimating a compression gainobtainable in the compressing, means for controlling said compressingdepending on said estimated compression gain.